1. Field of the Invention
This invention generally relates to systems, control subsystems, and methods for projecting an electron beam onto a specimen. Certain embodiments relate to a system that is configured to alter one or more characteristics of an electron beam during projection of the electron beam onto a specimen while the specimen is being moved at a non-uniform velocity.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
A lithographic process, as described above, is performed to selectively remove portions of the resist thereby exposing underlying areas of the specimen on which the resist is formed for selective processing such as etching, material deposition, implantation, and the like. Therefore, in many instances, the performance of the lithography process largely determines the characteristics (e.g., dimensions) of the structures formed on the specimen. Consequently, the trend in lithography is to design systems and components (e.g., resist materials) that are capable of forming patterns having ever smaller dimensions. A large part of the effort in developing advanced lithography systems involves the design and development of exposure tools that expose the resist in a predetermined pattern. In particular, the resolution capability of the lithography tools is one primary driver of lithography research and development.
Traditional lithographic processes utilize electromagnetic energy in the form of ultraviolet light for selective exposure of the resist. As an alternative to electromagnetic energy (including x-rays), charged particle beams have been used for high resolution lithographic resist exposure. In particular, electron beams have been used since the low mass of electrons allows relatively accurate control of an electron beam at relatively low power and relatively high speed. Electron beam lithographic systems may be categorized as electron-beam direct write (EBDW) lithography systems and electron beam projection lithography systems.
In EBDW lithography, the substrate is sequentially exposed by means of a focused electron beam. Such a lithography tool may be configured to scan the electron beam over the whole specimen in the form of lines, and the desired structure is written on the specimen by corresponding blanking of the beam. Alternatively, such a lithography tool may be configured to guide the focused electron beam over the regions of the resist to be exposed in a vector scan method. The beam spot may be shaped by a diaphragm. EBDW has relatively high flexibility since the circuit geometries are stored in a computer and can be optionally varied. Furthermore, very high resolutions can be attained by electron beam writing since electron foci with small diameters may be attained with electron-optical imaging systems. However, direct writing is disadvantageous in that the process is very time-consuming due to the sequential, point-wise writing. EBDW is therefore at present mainly used for the production of the masks required in projection lithography.
In electron beam projection lithography, analogously to optical lithography, a larger portion of a mask is illuminated simultaneously and is imaged on a reduced scale on a wafer by projection optics. Since a whole field is imaged simultaneously in electron beam projection lithography, the attainable throughputs can be markedly higher in comparison with electron beam writers. One disadvantage of conventional electron beam projection lithography systems is that a corresponding mask is necessary for each structure to be exposed. Therefore, the preparation of customer-specific circuits in small numbers is not economic, because of the high costs associated with mask production.
As described above, electron beam lithography (or e-beam lithography) can be used to generate patterns on wafers and reticles. In either case, the specimen must be moved during lithography such that the pattern can be printed at different positions on the specimen. Typically, e-beam lithography is performed in a step-and-write method in which a field is written on a specimen (e.g., on a wafer or a multi-field reticle) then the specimen is moved to a new field position. During the writing of the field, the specimen may be stationary. Alternatively, a number of new e-beam lithography tools write with a continuous motion/uniform velocity approach. In such approaches, a swath is written on the specimen in one direction at a constant velocity. The velocity is reversed after each swath such that each swath is written in alternating directions.
Both the step-and-write method and the continuous motion/uniform velocity approach have disadvantages. For example, in the continuous motion/uniform velocity approach, the stage must be decelerated and accelerated to reverse the velocity of the stage between swaths. Therefore, constant velocity must be re-established after the stage reversal such that writing of the next swath can be performed at the constant velocity. During such deceleration and acceleration of the stage, the stage must be moved such that when constant velocity is re-established, the electron beam is positioned at the correct position with respect to the specimen (e.g., proximate to the edge of the specimen) where the next swath begins. Therefore, the range across which the stage is required to move (including the length of a swath and distances for acceleration and deceleration) would need to be larger (and perhaps significantly larger (e.g., 2× larger)) than the dimension of the specimen in the direction of the swath. As such, the range of movement required for the stage may increase the size of the lithography tool and/or limit the minimum achievable footprint of the lithography tool.
Potentially more important is that both of these methods have an idle write time either while moving to the next field or during the time when the velocity of the stage is reversed between one swath and the next. Consequently, if the writing speed is not a limiting factor (depending on writing current, swath width, and resist sensitivity), then mechanical considerations may limit the duty cycle. In fact, if the maximum acceleration of the specimen is the main constraint on the achievable duty cycle, then the optimal duty cycle (for fastest writing) can be as low as 50% where half of the time is spent writing at a high velocity and half of the time is idle while the specimen is accelerated to reverse the velocity at the end of each swath.
Electron beam systems are becoming increasingly relied upon not only in lithography, but also in the inspection of devices formed in semiconductor fabrication. For example, as the dimensions of semiconductor devices continue to shrink with advances in semiconductor materials and processes, the ability to detect defects having corresponding decreasing dimensions has become increasingly important in the successful fabrication of advanced semiconductor devices. Therefore, significant research continues to focus on increasing the resolution capability of tools that are used to examine microscopic features and defects. Optical microscopes generally have an inherent resolution limit of approximately 200 nm and have limited usefulness in current manufacturing processes. Microscopes that utilize electron beams to examine devices, however, may be used to detect defects and investigate feature sizes as small as, e.g., a few nanometers. Therefore, tools that utilize electron beams to inspect semiconductor devices are increasingly becoming relied upon in semiconductor fabrication processes. For example, in recent years, scanning electron microscopy has become increasingly popular for the inspection of semiconductor devices.
Scanning electron microscopy generally involves scanning an electron beam over a specimen and creating an image of the specimen by detecting electrons that are reflected and/or scattered by the specimen. Scanning of the electron beam over the specimen may be performed as described above. Therefore, electron beam-based inspection systems may suffer from the same disadvantages described above. For example, e-beam inspection systems may have an increased structure size due to the range of movement required for the stage and may have a duty cycle that is limited by the idle time needed to reverse the stage.
Accordingly, it would be advantageous to develop methods and systems for electron beam lithography and inspection that reduce the range of movement of a stage that moves a specimen during scanning thereby lowering spatial requirements in the systems for the stage movement and that are capable of higher duty cycles than those that are currently achievable thereby increasing the throughput and lowering the cost of electron beam based methods and systems.